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Topic: Fire Pit Oven (Read 4770 times)

I have a weak electric oven that is unsuitable for high temperature baking and a gas grill which doesn't give me any heat from above. I do have a fire pit (steel body) that can burn wood and withstand the temperatures. I'm looking to build a structure that I can place on top of the fire pit when I want to bake pies and remove when finished. I've attached a 2D Google SketchUp drawing that shows my initial concepts. Since I didn't want to spend hours drawing an accurate version of my fire pit, I imported a stone one from the internet, but the concept should translate fine. My main concerns are with airflow and top-of-pizza heat radiation. It has already been suggested to me that I use a poor conductor for the bottom stone in order to retard bottom crust browning. Any inputs on materials or design are greatly appreciated. I assume that the airflow will go from the fire, into the oven, and out the loading 'port' in the front, but I'm not sure how to make sure that the flow goes over top of the pies.

In the drawing, the top surface and grates are steel, the bottom stone is fibrament, and the walls are brick.

That's why it's good to get your opinions; I hadn't given that any thought. I suppose I could shift the entire structure slightly to one side (left or right), before heating it up of course. Would an opening large enough to toss wood into the fire vent too much heat out of the fire? Do I need to create the opening from within the oven structure in order to minimize heat loss? I guess moving such a hot and heavy structure while hot is out of the question.

I guess moving such a hot and heavy structure while hot is out of the question.

Bingo.

Forgive the crude drawing, but one of the most common issues with these types of "ovens" is an imbalance of heat ie. too much from the bottom and not enough on top.

If I were to try and build this, I would try this first, and build a really hot fire for a few hours so the flames are kissing the top of the brick. That would give a fairly even heat distribution I would think.

This would probably be a little tedious to assemble/disassemble, but probably worth it IMO. I'm sure you'll get a lot of other great ideas too. Pics of your fire pit would help.

The insight is much appreciated JD. From what I see, your concept tries to thermally isolate the bottom stone from the fire as much as possible, while using both convection from airflow and radiation from the top insulating brick to speed cooking from above.

In terms of materials, will standard brick work well for what you label 'insulating brick'? Obviously, I cannot use standard bricks for the top surface. Scott replied to me over on Slice, suggesting Fibrament for the lower stone because of poor conductivity. This seems to jive with your suggestions, but I would like to avoid using such an easily crack-able surface for the main structure. I don't know what size certain kiln tiles/stones come in, but since they're not touching food they may be a viable option for the roof. I'm assuming that I should sandwich maybe 1/8 inch of steel between the grate and the lower stone in order to even the splotchy heat from the flame. I'll try to get pictures up soon. Thanks.

The insight is much appreciated JD. From what I see, your concept tries to thermally isolate the bottom stone from the fire as much as possible, while using both convection from airflow and radiation from the top insulating brick to speed cooking from above.

In terms of materials, will standard brick work well for what you label 'insulating brick'? Obviously, I cannot use standard bricks for the top surface. Scott replied to me over on Slice, suggesting Fibrament for the lower stone because of poor conductivity. This seems to jive with your suggestions, but I would like to avoid using such an easily crack-able surface for the main structure. I don't know what size certain kiln tiles/stones come in, but since they're not touching food they may be a viable option for the roof. I'm assuming that I should sandwich maybe 1/8 inch of steel between the grate and the lower stone in order to even the splotchy heat from the flame. I'll try to get pictures up soon. Thanks.

You'll get a lot of conflicting information on type of brick acceptable for use in open flames. A lot depends on how much money you're willing to spend and how detailed you want to be. Someone like me would use standard clay brick (non cementitious) for the entire project until I found a design I liked, and then moved on to firebrick where necessary.

But there are other variables to consider, for example you mentioned you want it to be removable which means mortarless. Is brick the best option? I don't know, probably not. Especially if you want to do Neapolitan. If you're after a quasi-Ny/Neo hybrid, I'd say that's entirely possible. There are a few members on the board doing mortarless ovens, you can do a search for that to get an idea what I'm talking about.

Finally, listen to Scott, he's the all-knowing oven/thermodynamics guy on the board.

Alright, I did a bit more SketchUp work. Unfortunately, my fire pit is shaped similar to in the drawing, like an inverted cone. This may prevent my from making the fire towards the rear of the oven only. The outer enclosure of bricks might be 2 bricks tall, with the inner enclosure being only 1 brick tall. The entire structure would probably be shifted atop the fire pit to allow wood to be dropped in from the sides.

Materials: Standard brick for the shown brick sections. I do not need 700 degree+ temperatures, 600-650 being fine.Fibrament or unglazed quarry tiles for the bottom hearth.Some sort of solid metal sheet between the bottom hearth and grate.

The top, whose material is still wide open for debate, is shaded translucent to allow us to see into the oven. Do they sell kiln supplies in maybe 2'x2' sizes for reasonable prices? I feel like Fibrament on the top would be prohibitively expensive due to the need for a large piece.

scott123

Finally, listen to Scott, he's the all-knowing oven/thermodynamics guy on the board.

JD, that is incredibly kind of you to say, but I'd really like to think that I'm a part of a group of forum members, that, collectively, is all-knowing in regards to oven/thermodynamics.

Dave, welcome to the forum. It's nice to continue our conversation on this side of the cyber fence There's some super smart people on Slice, but there's also some really sharp people here. Between the two, I'm sure you'll have this figured out in no time.

Okay, first off, what is the diameter of the firepit? Also, is the firepit sturdy enough to take a lot of additional weight? Do you think you could stand on it without a problem?

I could stand on it easily, all 135 pounds of me. I wouldn't put a 200 pounder on it though. I'll get better measurements and pictures tomorrow, but the top opening is about 36 inches with a 6 inch flap lip around the circumference. The bottom tapers down to less than a foot but the grate that the wood burns on is about 16 inches across.

I guess the limitations are that I really can't build a fire at the back of the pit specifically due to the small size of the fire grate, and the large amount of flex that may occur over a 36 inch diameter without super-rigid materials.

The one limitation I still worry about is the need to move anything around to feed the fire. If you were to go with my design, it is very "open", so it's not really much of an oven as it is a fire box. For the stone & dome to get hot enough, you would have to keep the fire very hot for a few hours and then keep a medium fire going while making the pizza. You don't need to be moving things around just to put wood on the fire in my opinion.

Bottom grate to top surface: ~14 inches (hard to measure)Edge of lip to edge of lip: 42 inchesTop opening: 30 incheswhich leaves for a 6 inch lip all the way around.

Weight on the actual structure should not be a problem as long as its roughly evenly distributed. It is not top heavy, per se, but I could not place 100 pounds of weight over one point on the lip.

I currently only have 6 left over pavers and a few bricks to work with. I have a cheap 14'' pizza stone that I feel would crack over any direct flame. I would like to keep costs down as much as possible, perhaps using unglazed quarry tiles for the bottom hearth. Performance does not need to be spectacular; I'm not trying to reproduce WFO results. I would be very pleased with 650 degree temps for the style of pizza I make and am fully willing to have to deal with turning the pie often for even cooking. 14'' pies will be about the largest I'd want to bake. Thanks.

Pictures in the next post hopefully. I can attach the photos fine, but when I try to submit I just get a white screen.

scott123

Alright, I have two scenarios, each with advantages and disadvantages.

As I look at the firepit, the thought occurred to me that any setup will be contingent on the lip of the firepit being completely heat proof. If the lip is painted, and heat will damage the finish, that means everything has to be built within the bowl, and I don't think that's feasible without drilling into/modifying the firepit. If, on the other hand, the lip is just a weathered finish/gun metal-ish patina to the steel, then we're good. But I just thought I'd point this out before we go any further. You might want to try burning a small piece of wood on the lip and see if any damage occurs.

Assuming the lip is flame proof, attached are my two proposed configurations (see below). Bear in mind, as you look at this diagram, neither has a ceiling. Both ovens have split firebrick (4.5" x 9" x 1.25") bases that are suspended by angle iron. I choose angle iron for expense. I don't know how much angle iron costs at Home Depot (probably not cheap), but, a lot of people have old metal bed frames lying around. If you don't have one or know of anyone who does, you might try Craigslist- either looking for ads, or posting a wanted ad of your own. You also might look into metal scrapyards and recyclers.

I'm basically covering up the whole grill, putting strategically placed bricks that can be slid out to add wood, and forcing the hot air through holes in the back of the baking chamber (which is not that different than the gap that JD had in his design).

The strength of the square version is the simplicity of the design and the lesser weight of fewer bricks. The disadvantage of the square version is that you have to take a round black stove pipe,

unbend it so that it's almost completely flat, and then put perfect 90 deg. corners to form the walls. I think it can be done, although I've never done it myself.

The advantage of a round oven is the easier fabrication of the round stove pipe wall. The disadvantage is the greater number of bricks/greater weight and the fact that the wood adding areas involve irregular brick shapes. You can try to cut these shapes out of bricks or you can cast them out of perlcrete. Perlcrete isn't inherently costly, but it can be tricky finding small bags of concrete, which may drive the price up. Both designs have gaps that have to be covered in order to force the hot air up through the baking chamber, so you have to find some kind of heat proof covering anyway, so perhaps if you're using perlcrete for that, you can use perlcrete for the irregularly shaped wood adding areas.

What you cannot use for any covering is aluminum, as it very likely will melt. Steel baking pans (usually teflon coated) might work, but they could warp, and, if they do, the hot air will escape in the wrong place.

One thing to bear in mind is that the angle iron supports will leave 1/4" gaps between the rows of firebricks, so you can't bake on the bricks directly. You will need to put another layer of quarry tiles on top of the bricks as your baking surface.

If you wanted to bake on the firebricks directly a far more elegant setup would be place the bricks on 1/16" to 1/8" steel sheet and support that with only two pieces of angle iron (no gaps). You'd need a steel sheet under the hearth and one under the wood adding area, and while 1/8" is pretty inexpensive, the number of cuts would probably make it cost prohibitive.

The ceiling can be either firebricks with angle iron running perpendicularly to the angle iron below, or, again, a more elegant, lighter, yet more expensive option would be 1/8" steel. You might be able to get a 20" square piece of 1/8" steel sheet for $50, but I don't know how much you're willing spend.

You could, in theory, go with pelcrete walls and bag the black stove pipe, but I'd like to see you with a material that can emit some heat, even if it is just thin steel.

unbend it so that it's almost completely flat, and then put perfect 90 deg. corners to form the walls. I think it can be done, although I've never done it myself.

I'm glad Kenji isn't here

I like the concepts. The square one seems to be more doable on a tight budget, as it doesn't require cutting brick or setting concrete. I am a bit confused however about the stove pipe. On your diagram, the blue line of stove pipe: is that length formed of pipe with the original circumference forming the sides (with height pi* diameter), or by the line of the pipe running down its length (which would be pi* diameter of length for the blue line). in other words, after picking an arbitrary size of tube, are you locked in for height, or for length?

Would flashing be a viable option if folded properly?

Is angle iron strong enough to hold a row of bricks without any perpendicular cross-bracing?

What are the black arrows?

I appreciate the time you're spending on the drawings and advice.

Logged

Dan

scott123

Re; stove pipe. You take a pipe, unclasp the seam holding it together so it's just a bent sheet of metal, and then, with tinsnips, cut the sheet into strips- the height of which is the desired height of your wall. You then take these strips and reclasp them to make as big a circle as you choose. The only length/height limitation is the pipe you buy and the size of the strips you can cut from it.

Roughly speaking, the circumference of the square baking area is 72 inches. The circumference (pi *diameter) of a 6" stove pie is 18.6. That means that 4 strips will give you what you need. A 24" long stove pipe will give you six 4" high strips or four 6" high strips, depending on what height you go with the wall (I'm thinking maybe 5" high, but it depends on your comfort level launching).

I just came up with the perfect way to do this. Black stove pipe. You can buy it at any hardware store in at least 6" diameter, possibly 8". This pipe is safe for high heat with no treatment needed. It will have a seam running it's entire length that is composed of a male end and a female end that simple snap into place. Say you want your ring 6" tall. Simply use tin snips to cut a series of 6 inch bands out of it. Then use the seams to connect them together to get the desired length to make a larger radius.

Now, I just took a look at my own stove pipe, and I see that it's riveted. If the one you buy is riveted, you won't be able to unclasp/reclasp, but you should be able to pop out the rivets (or maybe drill out) and replace them with nuts and bolts when you're putting the pieces back together.

The black arrows represent the sliding firebricks to temporarily remove and add wood.

The 1/8" gauge angle iron commonly used for bed frames is way strong enough to support rows of bricks without any cross bracing.

The square concept might require setting concrete. Those two holes on the side have to be covered with something, preferably something light, which is why I recommended perlcrete. You could just go with another column of bricks on each side, but that's 4 columns of bricks. That weight could add up, especially if you're using a brick ceiling. I haven't weighed a firebrick split recently, but they aren't light.

I recommend folded flashing for a KettlePizza knockoff because the weight being placed on top of it would have been pretty minimal. Unless you go with a thin gauge steel ceiling, which is going to drive up the cost, I'd stick with black stove pipe. Flashing, over the years, has lost of it's heft. On end, it could take a lot more weight than it could now. If you at home depot and want to see the kind of thickness of flashing they have, take a look, but I'd probably go with stove pipe for this application. Also, the flashing has to be copper, which isn't cheap, as, if you remember, aluminum in this configuration is a no go.

Btw, just an FYI, this oven should work perfectly fine without any kind of insulation with your bake time requirements, but under no circumstances can the bricks get wet. That means you have to have more than just a rain free bake window, but a rain free cooling window as well, that will allow the bricks to cool so that you can either cover it with a tarp or disassemble it.

a typical 23% alumina split would come in at 3.5 lb. The base setup of the square concept is 14 bricks, which is 49 lb. A 10 brick ceiling would add another 35 lb. to that. If you use bricks to cover up the sides, and add two more columns, that's 10 more bricks, 35 more lb.

That's 120 lb., not counting the weight of the angle iron or the quarry tiles.

For the ceiling, I can't imagine that stove pipe would be able to support bricks on end. Are you suggesting full firebrick cladding/walls to hold up a ceiling?

120 lbs is reasonable, but the cost of even 24 firebricks is a bit much. From what I see, they run about $24 for 6 bricks, so I'd be about $100 out just for 24. Are there any other materials that sacrifice thermal properties for price? I'm definitely using firebrick for the floor though.

To recap:

Floor: the angle irons supporting firebricks, covered with UQT.Walls: Stovepipe, perhaps slid in between the firebrick and irons on the sides, and in between two bricks at the back.Ceiling: Firebrick. Not sure what is supporting firebrick. I'm assuming there must be a support sheet under the bricks.Covering for the side openings: Would flashing work here? There would be direct flame contact.

I haven't purchased firebricks in a while, but $4 a brick sounds really high. Are you going to a brick yard? You want to go to a place that just sells bricks- wholesale, in huge piles- not a fancy schmancy retail outlet with a brick department. Also, are you pricing splits (1.25" thick) rather than full size (2.5" thick) bricks?

I believe almost all non stick cookie sheets are teflon on steel, but I'd check the specs first, if you have them. You can also feel if it's steel by it's heft- it will be thinner, but not that much lighter than aluminum cookie sheets.

The brick ceiling is exactly the same setup as the floor, except the angle iron is running side to side rather than back to front so that the stove pipe can support it.

Think about the force it takes to crush an aluminum can, the diameter of the can, and the gauge of the metal. Stove pipe is stronger metal, thicker gauge and the downward force is spread over a much wider circumference- 72" of it. If you made a 72" circumference 18"-ish diameter stove pipe that was only 5" high and put a piece of plywood on it, you could probably stand on it. What we're building won't be quite as strong because it will be missing a side, but it should be plenty strong enough to handle 10 firebrick splits and the angle iron supports.

It's depends on the quarry tile (I've come across really fragile and really durable quarry tiles), but you might be able to go with a UQT ceiling. Since it's not in contact with the food, you could also go with a glazed tile ceiling. I've been recommend black glazed tiles for broilerless oven setups. You could point the black tile down and gain some emissivity, but, for your target temps, you might not need it and could point it up. If you do go with a glazed tile, try to get one that has the darkest base color possible and isn't pure white. Also, if you go with glazed tiles, you'll probably want to double them up for a bit more thermal mass. 1/2" is as small of a thickness as you want to go.

Btw, a bit unrelated, but I just figured out another way to suspend a Kettlepizza knockoff ceiling that might be the cheapest of all, depending on what you drink. Aluminum soda cans might be tall enough to crimp together into a long wall, or, if they aren't long enough, 24 oz. aluminum cans. It's a bit more work than flashing, but, for some people, you can't beat free.

Btw, the more I think about it, if you were really careful about how high your fire goes, you might be able to pull a Kenji and use aluminum- on the walls, where, hopefully, they won't get much direct flame.

Hmmm. I'm wondering if you could run bolts up through the angle iron and use those to thread the ribbon of stove pipe through, kind of like the rollers in a film projector. That would give you more stability.

Lastly, I mention metal recyclers before- if you have a large one near you, go. It's amazing how much metal gets recycle. I went a few months ago to one near me and there was a load of stuff being tossed- perfectly good rebar, angle iron, some sheet. No promises, but you might find something that suits your needs. You won't be baking on it, so it doesn't need to be pristine clean, or if you do want it pristine, there's always naval jelly.

Only real question: I don't really see any 18'' stovepipe/chimneypipe. I believe you mentioned that if the pipe is snap-to-lock, then I should just be able to get, say, 8'' pipe and snap it together. And 5'' is a good height (length of pipe) to get?

I'm going to make a good drawing of the whole setup, take a bunch of screenshots, and post them by the weekend.Google isn't great for brickyards/scrapmetal, as they probably get their business mainly from contractors, but my dad probably knows some good places. I'll be calling around for split firebricks and scrap steel.